Selecting the right type of generator
Eskom has received many requests for information on the buying of
generators and how to connect their generators at their homes.
Generators are now available in a wide range of sizes. Generators with power supply
capacities of 5kW to 50kW are readily available in the personal and home use
markets, while industrial generators are anywhere from 50kW to over 3 Megawatts.
A genset is either petrol or diesel driven, and is mechanically connected to an
electrical generator. The engines need to be started either using a rope attached to
the engine similar to that of a lawnmower or with a starter motor and a switch.
Caution:
When a power failure occurs please keep candles and matches from
the generator fuel tanks.
The generator part of the genset will start generating power once the engine starts
and will stabilise at around 220 volts until a load or wattage is connected. This is
where it is vitally important to decide what load is to be connected to the genset.
Unfortunately, generator under sizing is one of the most common mistakes committed
by buyers. Not only could it damage your generator, but it can also damage other
assets connected to it, create hazardous situations, and even limit overall productivity
of the unit and/or the business relying on it. If nothing else, the key thing to remember
here is that more is always better than less.
Things to consider when buying a generator:
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Safety: Generators need to be correctly installed and/or connected to the building via a suitable and approved
change-over switch including installation of an over current protection circuit breaker in the main distribution
board by a certified electrician in terms of certificate of compliance SABS 1- 0142
Total electrical load to be connected
Voltage, frequency and phase
Does the generator need to start automatically? If so, then an automatic mains failure (AMF) mechanism needs
to be installed. If not, a procedure needs to be put in place for manual start and switch over.
Suitable location for the generator – outside or in vented area (not to be installed in an enclosed area or indoors)
Acceptable noise levels
Altitude and running time
1. How big a generator should you get?
This capacity depends on the sum (maximum total load expected at any given time) of the electrical loads you want
to power simultaneously, measured in watts. First, add up all the loads you know you want to be able to run
simultaneously. Then, as a precaution, figure out which electrical item in your house requires the most electricity to
start its motor and add that to your total, bearing in mind that the startup current of a pool pump motor, for example,
is about 1.4 times the running load and large items like air conditioners and some refrigerators can be two or three
times what they use while running. Make sure your generator can accommodate that extra electricity needed so that
larger items won't overload the system if they start up.
Every generator has two wattage ratings: running wattage and surge wattage. Generators are rated for surge
wattage because they should have some excess capacity in case the load you need is temporarily larger than what
you've calculated. When you buy a generator, choose the size based on the running wattage and its surge wattage
should automatically fall into line with what you need. If you're worried about needing more surge wattage, buy a
larger generator.
2. What loads should you consider powering with the generator?
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Kitchen, bathroom and sitting room lights
One plug in kitchen for kettle or cellphone charger
Refrigerator/freezer
Television and decoder
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Garage door opener
Microwave/oven
Computer
Security – alarm systems/electric fencing/lighting
3. How do you calculate all this?
All heating appliances such as stoves, heaters, dishwashers, geysers, kettles, tumble dryers, toasters and hairdryers
draw the most power. Controlling the number of lights switched on during generated power is advisable. PCs and
electronic equipment don’t use much power but are voltage sensitive. If the generator does not deliver a constant
voltage and there are dips and spikes in the system, equipment is likely to suffer damage. During generator use it is
best to switch off some appliances when using others.
Item
Quantity
Starting
Wattage
Refrigerator or freezer (Energy Star)
1
1200
Microwave oven
1
1000
Incandescent lights
6
360
Television
1
120
Computer
1
250
Garage door opener
1
1420
DVD player
1
350
Total
4700
Note: All wattages are estimates. Your application's exact wattage may vary.
Running
Wattage
192
1000
360
120
250
720
350
2992
In this example you need a generator with the following capabilities:
Recommended wattage: 3 692 (Appliance with the highest starting wattage + total running wattage for all other
appliances = total wattage needed)
Maximum wattage: 4700
Voltage: 220-240V
4. Do you need to hire an electrician to set up a generator safely?
A licensed electrician, who must carry out the installation, must understand and must have experience in wiring the
switches, since incorrect wiring of all the live, neutral and earth wires could be life threatening.
The local supply authority should be contacted to determine their particular requirements when a genset is installed.
You can check whether the electrician is registered by contacting the local offices of the Electrical Contracting Board
of South Africa. In addition always make sure that he gives you a valid electrical certificate of compliance. NEVER
attempt this work yourself and never plug a supply from a genset into a socket of your house. If you do not want to
install a changeover switch, a temporary extension lead from the genset can be used to plug in the equipment that
you require. The lead size is important to ensure that it can carry the required load.
The transfer or change-over switch is likely to be a manual device which has three switching positions - most have a
central off position and a position to the left and right of the off position. One is for the main supply and the other
for the generator supply.
This will mean that when the main supply fails, the switch will have to be manually turned from mains to the
generator supply. The whole process will have to be repeated once the main supply returns.
This “switching” procedure is a vital part of the process as a “back feed” is both potentially dangerous and can also
damage the generator and electronic devices such as computers, TV and hi-fi. The main supply and the
generator supply must never run in parallel. These transfer switches can also be automatically controlled,
but this is far more complicated and costs a great deal more.
5. Can't I just plug my generator into a wall outlet?
No. This is known as backfeeding, and it's very dangerous for a variety of reasons. For instance, if someone forgets
to throw the main circuit breaker to electrically isolate the house from the grid, then the generator could send
electrical power beyond the house and out onto the grid. Also should the earth leakage or main switch be left on,
when grid power resumes it will destroy your generator and could even cause electrical or other fires and many other
hazards. In addition, if you choose to switch off the earth leakage main switch to avoid this issue, you need to
remember that your family is at high risk of electrocution as when back-feeding power to your house, since the
electrical system will not have earth leakage protection.
6. What's the difference between a standby generator and a backup generator?
A standby generator is permanently installed apparatus, much like a compressor for a central air-conditioning
system. Its engine runs on natural gas or propane. A backup generator is a small, petrol-engine generator that you
wheel into position outside the house and then plug into the transfer switch. Or it can be connected to electrical loads
via heavy-duty extension cords.
7. If it's raining outside, can you put the generator in the garage and run it there, as long as the
door stays open?
No. Never run a generator inside a house, inside a garage, under a
carport, on a porch, inside a screened porch or near an open window.
Even with the garage door open, the carbon monoxide (CO) in the
generator's exhaust can sicken somebody inside the house or, in the
worst case, even kill someone.
8. What other safety tips should you keep in mind?
Have working
smoke and CO
detectors in the
house when using
a generator.
Keep the generator at least 5 metres from the
house to minimize risks from CO and also the
risk of the generator's hot muffler melting vinyl
siding.
Never fuel a generator while it is operating
(running) or when it's hot as it is a huge
fire/explosion hazard.
Remember: "Let it cool before you fuel."
9. Generators are loud. What can be done about that?
Unfortunately, not much. More mechanically advanced generators do a better job than older ones at adjusting engine
rpm to their electrical output. This reduces their running speed, which is quieter and conserves fuel. The simplest
way to reduce generator noise is to reduce the electrical load you're imposing on it.
10. Does the generator need to be grounded?
Follow the instructions in the owner's manual. If the manual calls for grounding the generator, that's relatively easy to
do. One simple way is to run a 12-gauge ground wire from the grounding terminal on the generator to a copper
ground rod you've driven into the soil next to the generator. (The generator will have a grounding terminal symbol to
help you identify the terminal's location.)
As an alternative, the manual may ask you to run a ground wire from the generator's grounding terminal to the
ground bus inside the house's service panel. As long as you follow the instructions provided in the manual, the
generator will be safely grounded.
Power consumption chart
This chart is provided an example as to how wattage varies between various electrical devices. It is not
meant to be a strict guide to calculate your requirements. For the most accurate calculations refer to the
owner’s manual of each device, tool or appliance, or consult a professional electrician:
Home applications
Refrigerator or freezer (Energy
Star)
Microwave oven
650 watts
800 watts
1000 watts
Incandescent lights
Television
Tube type
Flat screen (20”)
Flat screen (46")
Coffee maker (4 cup)
Dishwasher (cool dry)
Electric fry pan
Electric stove (8-inch element)
Automatic washing machine
Tumble dryer
Approximate starting wattage
Approximate running wattage
1 200
132-192
1000
1300
1500
as indicated on bulb (i.e.
60W)
1000
1300
1500
as indicated on bulb (i.e
60W)
300
120
190
600
540
1500
2100
1200
6750
300
120
190
600
216
1500
2100
1200
5400
240V required?
Y
Y
Radio
Computer
Laptop
Desktop
Monitor (LCD style)
Printer
Geyser
Garage door opener
50 to 200
50 to 200
200-250
600-800
30
400-600
4500
1420
200-250
600-800
30
400-600
4500
720
Y
Typical sizes needed
Small portable: 3 – 4kW
What it can power: The basics, including items such
as...
 Refrigerator (600 watts)
 Microwave (1 500 watts)
 Sump pump (600 watts)
 Several lights (400 watts)
 TV (200 watts)
Large portable: 10kW
What it can power: Adds choice of...
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Small water heater (150 litre -200 litre hot water
geyser – 3 000 watts)
Central air conditioner (5 000 watts)
Electric range (5 000 watts)
Compiled by Eskom Corporate Affairs
February 2015
www.eskom.co.za
Midsized portable and small stationary: 55 – 8kW
What it can power: Same as small models, plus...
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Portable heater (1 300 watts)
Computer (250 watts)
Heating system (500 watts)
Second pump (600 watts)
More lights (400 watts)
Large stationary: 10 – 15kW
What it can power: Same as large portable models,
plus...
 Clothes washer (1 200 watts)
 Electric dryer (5 000 watts)